Protective internal coatings for porous substrates

ABSTRACT

A material contains open pores in which the channels and pores that are internally coated with at least one layer of phosphorus-containing alumina. Such material is formed by infiltrating a porous material one or more times with a non-colloidal, low-viscosity liquid coating precursor, drying, and curing the coating precursor to form a phosphorus-containing alumina layer within pores of the material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part to International ApplicationPCT/US12/27355, filed Mar. 1, 2012, which claims priority to U.S.Provisional Application No. 61/448,268, filed Mar. 2, 2011, allincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made, in part, with government support under ContractNo. N68335-09-C-0213 and Contract N68335-08-C-0015 each awarded by theU.S. Department of Defense. The U.S. government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

This invention relates to internal coatings for substrates with openpores and more particularly relates to metal oxide based coatingsapplied to porous advanced materials for use at high temperatures or incorrosive environments.

Typically, advanced high performance materials must remain stable andretain structural integrity over a wide range of environmental andtemperature conditions. Many such advanced performance materialscontain, or are attached to, a surface substrate containing open poresand channels, which act as a thermal or corrosion barrier with respectto underlying materials. Examples of use of such high performancematerials include components incorporated into turbine engines such asused in aircraft, aerospace, and energy and power generationapplications. In such a turbine, hot combustion gasses contactcomponents such as rotors, turbine blades, vessels, and shrouds. Otherapplications of high performance materials include ceramics, ceramicmatrix composites (CMC), plasma-sprayed ceramic coatings, andrefractories.

Materials and articles containing inorganic porous surfaces are usedextensively in many industrial applications to provide thermalinsulation, heat resistance, reduced weight, or increased toughness.Such a porous surface material may be metallic or ceramic. An example ofsuch an article is an external coating of a porous material on anunderlying metallic or ceramic material. Typical porous industrialarticles contain open pore channels of varying sizes (dimensions ofseveral micrometers down to nanometers in pore diameter) and geometries.These open pore channels often compromise the chemical, mechanical, andelectrical properties of these articles. An aspect of this invention isan internal coating within a pore structure that can provideenvironmental protection and can maintain or improve the chemical,mechanical, and electrical properties of such articles

Specific applications of high performance materials or substrates usefulin this invention include, but are not limited to, thermal barriercoatings, thermal protection systems, radiation shielding systems, heatrejection systems, lightweight thermally stable structural members andsystems for active or passive functionality, environmental barriercoatings and systems, anodized metals, metallurgical overlay coatings,plasma spray coatings, ceramic metal composite coatings, electron beamphysical vapor deposition derived coatings, and slurry coatings.Articles embodying these applications may include, but not limited to,refractory ceramics and composites, turbine engine components, exhaustor airframe components, and other aerospace or utility land-based powergeneration turbine hardware.

A purpose and function of these high performance materials is based inthe high stability of the materials to harsh environments such as toextremely high temperatures that may be above 500° C., typically above800° C., and especially above 1000° C. Typical harsh environmentsinclude an oxidizing, reducing, high temperature, or vacuum atmosphere,and additional atmospheric components such as water (vapor or liquid),and common atmospheric contaminants such as dust, dirt, sand, ash, fueladditives/contaminants or biofuel derivatives, and various organiccompounds. Further, porous ceramic materials or coatings used atelevated temperatures for extended periods may experience sintering inwhich pores coalesce. Such sintering may result in variation of thermalinsulation properties and additional stress that may degrade theintegrity of the material. Added requirements imposed by such harshenvironmental conditions associated with the high temperatures aredirected to stability of the materials with respect to conditions, suchas oxidation, corrosion, embrittlement, fatigue, mechanical wear, stresscracking, structural changes such as sintering or densification, loss ofadhesion or loss of material (mass or thickness), reduction, andchemical reaction.

Pore coalescence is the accumulation of multiple pores to a commonproximity and subsequent union of the pores to form a lesser number oflarger pores. This process is favored when the mobility of pores withina material is sufficiently high to allow the pores to accumulate inproximity to one another; the joining step then will occur quickly oncethe proximity of neighboring pores is sufficient. This process can besubstantially impeded through increased bonding within the bulk of amaterial, which can be accomplished through heat-treating orreaction-based doping processes.

A grain is an amount of material that exhibits a consistent phase andcrystal structure orientation over some finite scale, typically on theorder of 10 nanometers to 100 μm, and often referred to as acrystallite. Polycrystalline materials are comprised of a multitude ofsmall grains. Grains comprise all ordered materials, including ceramics,metals, and many polymers.

Grain growth is the process of the joining of multiple small grains,which are in proximity to one another to form a lower number of largergrains. This process is favored when grain boundary mobility issufficiently high to allow grain boundaries to impinge into neighboringgrains. Grain growth processes decrease porosity and impart mechanicalstresses into the bulk of the material, which often have detrimentalconsequences, often resulting in the destruction of intentionallyengineered material architectures and stress fields.

Other porous materials that experience corrosion and sintering at hightemperatures include refractory materials, such as used in steelmanufacture or used in furnaces, kilns, reactors, or incinerators. Atypical refractory is a metal oxide such as oxides of aluminum,magnesium, calcium, titanium, zirconium, and chromium. Other refractorymaterials include silica, silicon carbide, and graphite. A refractorymaterial may contain mixtures of refractories.

A thermal barrier coating may be prepared from a ceramic material, suchas a chemically (metal oxide) stabilized zirconia. Examples of suchchemically stabilized zirconia include yttria-stabilized zirconia,scandia-stabilized zirconia, calcia-stabilized zirconia,magnesia-stabilized zirconia, and combinations thereof. The thermalbarrier coating of choice, typically, is an yttria-stabilized zirconiaceramic coating. A representative yttria-stabilized zirconia (YSZ)thermal barrier coating usually contains about 7 wt. % yttria and about93 wt. % zirconia. The thickness of the thermal barrier coating dependsupon the metal part or component it is deposited on, but is usually inthe range of from about 0.5 to about 2 millimeters (typically 0.1 to 1mm) thick for high temperature gas turbine engine parts.

To prevent turbine components from getting too hot, thermal barriercoatings (TBC's) often are coated onto various surfaces of the turbinecomponents to insulate the components from the high temperatures in thehot gas path. TBC's are an increasingly important component in currentand future gas turbine engine designs because of the higher operatingtemperatures in gas turbine engines. Examples of turbine engine partsand components for which such thermal barrier coatings are desirableinclude turbine blades and vanes, turbine shrouds, buckets, nozzles,combustion liners and deflectors, and the like. These thermal barriercoatings typically are deposited onto a metal substrate (or moretypically onto a bond coat layer on the metal substrate for betteradherence) from which the part or component is formed. The TBC reducesheat flow and limits the operating temperature experienced by such metalparts and components. A suitable metal substrate typically is a metalalloy such as a nickel-, cobalt-, and/or iron-based alloy (e.g., a hightemperature super alloy).

Although significant advances have been made in improving durability ofthermal barrier coatings on metal substrates such as for turbine enginecomponents, these coatings still are susceptible to various types ofdamage, including objects ingested by an engine, erosion, oxidation, andattack from environmental contaminants. In addition, in trying toachieve reduced thermal conductivity, other properties of the thermalbarrier coating can be adversely impacted. For example, the compositionand crystalline microstructure of a thermal barrier coating, such asthose prepared from yttria-stabilized zirconia, can be modified toimpart to the coating an improved reduction in thermal conductivity,especially as the coating ages over time. However, such modificationsalso can unintentionally interfere with desired spallation resistance,especially at the higher temperatures that most turbine componentsexperience. As a result, the thermal barrier coating can become moresusceptible to damage due to the impact of, for example, objects anddebris ingested by the engine and passing through a turbine sectionsthereof. Such impact damage eventually can cause spallation and loss ofthe thermal barrier coating.

Chemical-based pore obliteration can occur when a porous material isinvolved in chemical reactions with an externally introduced reactant,which often is a contaminant species. Because environmental conditionsand contaminants are present in widely variable ranges, a great numberof possible reactions are common in advanced material applications andare magnified with increasing temperatures. A common reactant forthermal barrier coatings, for example, is a calciumaluminosilicate-based glassy material, which is found as a constituentin sand and dust. The starting sand and dust material is found commonlyin the atmosphere in areas around the world, and may be ingested intoturbine engines on aircraft as well as stationary power generatingequipment. Sand or debris is melted in the high temperature environmentinside a turbine engine, and the resulting molten material is depositedonto hot surfaces of internal structures of a turbine engine, which arecoated with porous thermal barrier coatings. Molten sand debris, thepredominant constituent of which is a slag-like and glassy mix ofcalcium-magnesium-aluminosilicates (referred to as “CMAS”), infiltratesinto porous structures of a thermal barrier coating upon contact.Ceramic oxide-based thermal barrier coatings typically are very reactivewith CMAS compositions and, essentially, dissolve into solution in theCMAS. Reaction products are precipitated from the dissolution melt thatare more stable than the original CMAS+TBC mixed system. Thus, theoriginally designed porous architecture of the TBC is eliminated as thechemical reaction with CMAS progresses and functionality of the startingTBC material declines.

Solidified CMAS causes stresses to build within the thermal barriercoating, leading to partial or complete delamination and spalling of thecoating material and, thus partial or complete loss of the thermalprotection provided to the underlying metal substrate of the part orcomponent.

Pores, channels, or other cavities that are infiltrated by such moltenenvironmental contaminants can be created by environmental damage oreven the normal wear and tear that results during the operation of theengine. However, the porous structure of pores, channels, or othercavities in the thermal barrier coating surface more typically is theresult of processes by which the thermal barrier coating is depositedonto the underlying bond coat layer-metal substrate. For example,thermal barrier coatings that are deposited by air plasma spraytechniques tend to create a sponge-like porous structure of open poresin at least the surface of the coating. By contrast, thermal barriercoatings that are deposited by physical (e.g., chemical) vapordeposition techniques tend to create a porous structure comprising aseries of columnar grooves, crevices or channels in at least the surfaceof the coating. This porous structure can be important in the ability ofthese thermal barrier coatings to tolerate strains occurring duringthermal cycling and to reduce stresses due to the differences betweenthe coefficient of thermal expansion (CTE) of the coating and the CTE ofthe underlying bond coat layer/substrate. CMAS mitigation coatings oftenare needed for gas turbine operation above 1200° C. Many turbine enginesoperate in this temperature regime. Without CMAS mitigation,functionality of a TBC often is compromised, and the component may fail.Coatings of alumina have been used to mitigate CMAS attack; however,this approach has not been successful largely due to poor distributionof alumina throughout the porous network. Addition of alumina to theCMAS increases its melting point, thereby arresting its molten flow and,consequently, its degrading effect. Thus, in this case, the alumina actsas a sacrificial layer to arrest further flow or degradation, whicheffectively protects the substrate from CMAS attack.

In general, most porous ceramic coatings and materials are designed toretain porosity to exploit mechanical properties (toughness), lowdensity (e.g., in aerospace/transportation), filtration, and thermalinsulation; however, these porous coatings are prone to environmentaldegradation. There is a need selectively to deposit or plug the porositywith an inert material without substantially altering the overallporosity level or microstructure to retain the properties. Also, thereis a need to protect pore walls from reacting with environmentalspecies. Such protection needs to be done preferably with a hightemperature stable material that is inert, chemically compatible withvarious substrate materials, and remains morphologically stable uponlong-term high temperature exposures.

Porous ceramic materials and composites also may exhibit lowermechanical strengths compared to their higher density counterparts. Suchlower strengths are partially attributable to micro-scale flaws (on porewalls or interiors) contained within porous microstructures. Theinternal coatings of the present invention may heal such flaws andprovide an improvement in mechanical strength and toughness of theporous substrate material. External porous coatings, such as thermalbarrier or insulation coatings on a structure also are subject to severeerosion in harsh environments. Internal pore coatings of this inventionmay improve performance of these external coatings under erosionconditions.

Anodized aluminum is commonly used in many consumer and industrialproducts to impart corrosion protection. However, anodized aluminum hasa thin amorphous alumina surface that contains micro-scale open porositythat serves as pathways for moisture, salt, or other environmentalcontaminants to penetrate and attack the underlying non-anodized metal.There is a need to seal at least partially the open porosity in thesecoatings to enhance their corrosion protection.

Porous materials and coatings also are used as dielectric materials orto provide electrical insulation in many use applications. Presence ofopen pore channels often may compromise the durability of the materialsover time due to ingress of environmental species that alter theelectrical properties either by the presence of such species or throughreaction with pore walls. There is a need for a system that creates longterm stability during operation and service of porous materials tomaintain their designed electrical properties.

There is a need for an effective system to protect porous advanced highperformance materials against harsh environmental conditions experiencedduring use without significantly affecting physical properties of suchmaterials such as strength, weight, thermal conductivity, weartolerance, stress cracking resistance, and the like. Further, there is aneed to minimize sintering in porous materials at elevated temperatures.

Conventional attempts to mitigate harsh environmental conditions onporous advanced high performance materials include placing a coating ora metallurgical overlay coating on the exterior surface to completelyfill pores of such materials. However, such exterior coatings (e.g., atopcoat) tend not to be hermetic or tend to crack or spall duringthermal cycling such that the sealing is not effective or durable. Otherknown protective techniques, such as use of nanoparticle-based aluminaslurries to infiltrate fine pores of a thermal barrier coating, do notyield conformal coatings on the pore walls and do not produce sufficientenvironmental protection. Use of metal organic chemical vapor deposition(MOCVD) alumina coatings has been described to infiltrate the finerpores in thermal barrier coatings. However, these coatings have highcost and may not be suitable for large complex-shaped components, andsuch coatings may not have an ability to infiltrate fine pores withoutadversely affecting TBC properties such as strain tolerance. Further,this technique may not be suitable for use in the field or for repair.Slurry-based systems to deposit alumina-based films into a poroussubstrate may not be suitable to deposit conformal films on pore wallsor may not penetrate very small pores or uniformly infiltrate throughthe depth of the porous substrate and, thus, may not provide adequateenvironmental protection. Presently, there are no practical methods ofdepositing clear, low viscosity, high yield solutions to form goodquality alumina-based films onto pore walls of a porous substrate.

This invention provides a solution to providing an internal coating of aporous substrate to form a superior environmental protection system,that further is capable of maintaining or improving chemical,mechanical, and electrical properties without significantly affectingdesirable properties of a porous advanced material. A pore wall barrierinternal coating and selective blocking of mesopores may be createdwithout having a detrimental effect on the design properties of suchmaterials.

SUMMARY OF THE INVENTION

A material contains a porous substrate containing open pores andchannels that are internally coated with at least one layer ofphosphorus-containing alumina. Such material is formed by infiltrating aporous material one or more times with a non-colloidal, low-viscosityliquid coating precursor, drying, and curing the coating precursor toform a continuous phosphorus-containing alumina layer within pores ofthe material.

DESCRIPTION OF THE INVENTION

Typical high performance materials useful in this invention are based ona porous (non-fully dense) structure. The architecture of thesematerials may be designed for the application for which the advancedmaterial will be used, such as providing a sufficiently low thermalconductivity structure joined to another material to permit use of othermaterials at elevated thermal or radiation conditions. Pore structurestypically dominate the proportion of free surface area of thesematerials, with common surface area proportions that are associated withpores alone being 90% or higher, often 98% or above, depending on theoverall material thickness and roughness of the externally free surfaceof the bulk material structure.

The presence of pores including open and closed pores within highperformance materials serve various purposes, and can be the result ofintentional processes to achieve pore formation, or unintentionalprocesses that result from methods of forming or creating the materials.A function of the pores typically includes increasing mechanicaltoughness (e.g., resistance to fracture propagation under dynamicloading), decrease in weight, and increase in resistance to thermalconduction and fatigue at elevated temperatures during service. Presenceof pores in advanced materials in extreme temperature environments iscritical for effective performance, and maintaining the designed porestructure is paramount to the determination of the effectiveness anduseful lifetime of those materials during normal operation. Thus,protection and maintenance of the initially designed pore microstructureis important in order to achieve the design function for such advancedmaterials over a sufficient useful lifetime.

Porous substrates that are subject to extreme temperature variationduring service are subject to stress fracture. Typically, stressfracture may be caused by thermal expansion and contraction of a porousmaterial due to thermal cycling in service. If such a porous substrateis coated with a material that passivates the surface of the substrate(typically referred to as a topcoat), repeated expansion andcontractions of the underlying porous substrate may cause cracks in thesurface coating or the surface may be subject to erosive or abrasiveconditions in a service environment which compromises the integrity ofthe topcoat. Cracks in a surface coating will expose the underlyingporous substrate to environmental conditions such as heat, contaminants,and oxidizing gases, which may degrade the substrate properties.However, coating interior surfaces of a porous material in accordancewith this invention will not be subject to degradation from such thermalstresses.

Pores are macroscopic-scale voids in a material characterized by aninternal surface area interfacing with a gaseous environment. Pores areinherently unstable in a thermodynamic sense because the internalsurface area contains stored potential energy that, when driven off,creates a more stable material (i.e., having less stored energy).Prevailing mechanisms for pore deterioration include sintering, graingrowth, chemical reaction based pore obliteration, and pore coalescence,the driving force of which is minimization of overall energy throughreduction in surface energy through pore eradication. Elevatedtemperatures applied to a porous material promotes pore deteriorationbecause the activation energy required for such destructive process isreduced in proportion to the temperature increase, which is aparticularly challenging issue for advanced materials used at hightemperatures. According to the present invention, a conformal coatingapplied to inner surfaces of open pores resists sintering and therebypromotes retention of the original pore structure, which providesdurable performance in use at elevated temperature or under otherenvironmental conditions.

Sintering processes (i.e., thermally driven densification) occurnaturally as a result of entropically favored overall energy statereduction and are increased substantially as temperatures increase. Theresult of sintering is the reduction in the size of pores in a bulkmaterial over time, yielding an increasingly dense bulk material withminimum total overall surface area (internal and external freesurfaces). Sintering induces growth stresses, which lead tomicrocracking and material degradation in service especially underthermal cycling conditions.

An aspect of this invention is forming a thin, protective layer of aphosphorus-enriched alumina or phospho-alumina onto the interiorsurfaces of a porous material with open porosity (i.e., having pores inopen communication with the exterior surface of the material). An effectof such interior layer (or a combination of sublayers) is to provideprotection against harsh environmental conditions that would causecorrosion or oxidation of the material. A further benefit of suchinterior coating layers is to maintain physical or mechanical propertiesof the material such as thermal conductivity, stress crack resistance,electrical properties, and weight reduction. Depending on the specificuse of the material, some benefits may be more advantageous than othersin practice. For example, maintenance of weight reduction may be moreimportant than thermal conductivity in some applications. In any regard,porous materials with interior coatings produced through this inventionmay demonstrate one or more other advantageous properties such asmaintenance of stress crack resistance and corrosion inhibition.

Typical porous material substrates useful in this invention are shapedarticles formed from substances such as a metal, metal oxide,refractory, ceramic, or ceramic matrix composite materials. Thesearticles are solid inorganic materials that contain structures withinternal pore spaces or channels that are in open communication to theexterior surface of the material. Porous substrates include a ceramicmaterial or coating, a ceramic matrix composite, or a metal or metalalloy. Typical porous substrate materials used in this inventiontypically are metal oxides and carbide/nitrides (ceramics), but mayinclude elemental materials such as metals and carbon (graphite), aswell as composite materials from carbon, metal, or particulate orfiber-reinforced metal or ceramic composites and metallic foams.Especially useful are oxides of aluminum, zirconium, hafnium, tantalum,titanium, chromium, nickel, silicon, yttrium, and cerium, and the metalsof titanium and titanium alloys, nickel and nickel, cobalt, and ironbased superalloys, tungsten, rhenium, and hafnium, as well as metals ofaluminum, titanium, chromium, nickel, iron, and cobalt in the form ofsacrificial oxide formers, and silicon carbide and silicon nitride,fused silica, aluminosilicates, perovskite oxides (ABO₃ repeating cellblocks), pyrochlore oxides (A₂B₂O₆ and A₂B₂O₇), and carbon. Typically,suitable porous substrates suitable for use in this invention may beinorganic or organic structures that contain open pores and that arestable at curing temperatures used to form the phospho alumina internalcoatings of this invention.

A benefit of the invention is a protective barrier coating within aporous structure (such as a barrier against oxidation or othercorrosion) which is securely bonded to the interior surfaces of thesubstrate and which remains an effective barrier or passivation coatingafter prolonged environmental exposures such as prolonged exposure tohigh temperature in an oxidative environment. Typical harshenvironmental conditions to which coated substrates of this inventionmay be exposed include temperatures above 800° C., typically above 1000°C. and may exceed 1200° C. and further exposure to corrosive materialssuch as gases, including but not limited to, oxygen, water vapor,hydrogen, carbon monoxide, and other chemical substances such as moltenCMAS. Typical internally coated porous materials on substrates formed inaccordance with this invention are expected to maintain corrosionresistance for hundreds to thousands of hours at high temperatureoxidizing conditions. An internally coated porous materials onsubstrates formed in accordance with this invention will slow CMASinfiltration and, thus, prolong the useful service life an articlesubjected to CMAS attack. In the present invention, alumina, in the formof phospho-alumina coating is distributed more uniformly throughout theporous network, thus providing a “local” source of alumina to CMAS formore effective protection. As CMAS flows through the pore channels, CMASinteracts with phospho-alumina, whereby alumina content in CMAS isincreased which raises the melting point of the CMAS-containing mixture,thus arresting further flow. This effect is particularly important inmesopores because mesopores comprise the bulk of open porosity surfacearea in YSZ TBCs and hence the importance of coating mesopores.

Porous material substrates useful in this invention typically containpores with interior channels that have communication to the surface ofthe substrate. Such channels may be interconnected and may containadditional pore structures, all of which form a part of a poroussubstrate material containing pores. The interior pores may have varyingwidths or sizes and typically may range from 5 micrometers (5 μm) ormore to 30 nanometers (nm) or less. These pores typically have anirregular cross-section with diameters measured as mean (average)diameters. More typically, porous substrates useful in this inventioncontain an open pore network (i.e., with open communication with thesurface) with channels with diameters of up to 2 micrometers, typicallyup to 1 micrometer, many times up to 200 nm or up to 100 nm. Some porechannels usually have diameters greater than 2 nm (typically greaterthan 4 nm) and may have diameters of 30 nm or less, and may be up to 50nm. Typical pore networks have channels ranging from 200 nm to 20 nmwith a majority of the channel volume in channels having diameters of 30to 100 nm. Under IUPAC nomenclature “micropores” have diameters lessthan 2 nm, “mesopores” have diameters ranging from 2 to 50 nm, and“macropores” have diameters ranging from 50 to 1000 nm (1 μm) (althoughas used in this invention a macropore is considered to range up to 2000nm (2 μm)). Thus, a typical suitable porous substrate material useful inthis invention contains a substrate containing an open pore network witha combination of mesopores and macropores. Such a substrate materialalso may contain pores with mean pore diameters up to 3 or up to 5micrometers.

Pore volume or porosity may be measured by the Archimedes displacementmethod. Pore volume and surface area may be measured using the BETtechnique or other suitable techniques such as helium pycnometry.

Pores contained in porous materials useful in this invention (which maybe mesopores and macropores) refer to pores and interconnected channels,which typically form a complex web of internal spaces or voids ofvarying dimensions and form. Typical forms of pores are extended tubesand spaces of varying dimension and cross-sectional shape connected in anetwork. A porous material with open pores contains pores with opencommunication with the surface of the material. The terms “pores”,“channels”, or “pore channels” may not necessarily be descriptive of theactual shape of the internal porous structure of a porous material, butare used to describe the network of internal spaces contained in suchmaterials and collectively described as pores. Although main channelsmay be 2 to 0.1 micrometers (2000 to 100 nm) in mean diameter, there maybe very fine pore channels connected to such main channels that havemean diameter dimensions of less than 50 or less than 20 nanometers.

In typical porous materials used in this invention, the pore volume andsurface area represented by the mesopores may be as great or greaterthan the pore volume and surface area represented by the macropores.Thus, protection of the mesopores from environmental degradation isimportant to maintaining the desired beneficial physical properties ofsuch porous materials. In porous material substrates useful in thisinvention internal pore volume typically may be at least 1% of the grossvolume of the substrate and may be up to 60% or more of the grossvolume. In a typical porous substrate the internal pore volume is atleast 5 or 10% of the gross volume and may be up to 40 or 50% of thegross volume. Also in a typical porous substrate useful in thisinvention the ratio of mesopores to macropores typically may range from1 to 10 to 1000 to 1. Typically, mesopores constitute 25% or more of thesurface area of the substrate and may range up to 75% or up to 99% or upto 99.99% of the surface area and may be 30 to 60% of the surface area.However, mesopores may constitute 25% or more of the porosity of thesubstrate and range up to 75%, or up to 99% or up to 99.99% of themeasured porosity and may be 30 to 70% of the measured porosity.

In one aspect of this invention, substantial porosity of internallycoated substrates is maintained. Many substrates useful as highperformance materials have open-cell porosity, which can be measured asaverage pore volume. Because such porosity is important to theusefulness of such high performance materials (such as heat barriers), acoating that functions to protect the substrate against oxidativedegradation should not significantly affect the porous character of thesubstrate. Typically, there is less than 25% or less than 10%(preferably less than 2% and may be below 1%) change in pore volumeafter application of a coating of this invention. Thus, typically, 75%(on a volumetric basis), 90%, or more of measured porosity is retainedbased on an uncoated substrate. Preferably, at least 95% of the porosityis retained and porosity retention may be at least 98% or at least 99%.Thus, a superior internal coating is thin (typically less than 2micrometers, preferably less than 1 micrometers, more preferably lessthan 0.5 micrometer) and is able to coat surfaces within pores of asubstrate and protect all surfaces of a porous material againstoxidative degradation at extreme operating conditions experienced bysuch substrate including temperature and moisture and contaminantconcentrations.

As used in this invention, maintenance of porosity or weight gain aftercoating refers to the top segment of a porous material, which may be upto 5 mm thick. For example, a refractory material (e.g., a brick) mayhave porosity throughout a substantial body, but an environmentalcoating need only to penetrate a few millimeters into the body toprovide protection. However, in many applications the porous substratemay be substantially thinner such as in TBC's and CMC's.

In another aspect of this invention, a high performance porous materialcontaining open pores is internally coated with a continuous layer of aphosphorus-containing alumina according to this invention to an extentthat open or interconnected porosity of the coated substrate is at least90%, preferably at least 95% and typically at least 98% or more, of theporosity of an uncoated substrate.

In another aspect of this invention, larger interior spaces within asubstrate that are accessible only by smaller channels (e.g., mesoporesize channels) may be coated with one or more layers ofphosphorus-containing alumina. This is in contrast with conventionalcoating systems in which such large interior spaces could not be reachedby an external coating.

In some uses of this invention, a substrate may contain large opensurface pits or cracks that could be coated with an external coating,but smaller mesopores or macropores connected to such pits or crackswould not be coated in a conventional system, but these smaller openpores and channels may be coated internally using this invention.

In an aspect of the invention, multiple layers of phospho-alumina aredeposited on internal surfaces of a porous substrate. Creation of suchlayers is a result of multiple treatments of the precursor solution,preferably under pressure differential (preferably vacuum) conditions.Although observation of separate layers in the internal structure isdifficult, the thickness of the resulting deposited material increasesas the number of layers increase in some areas of the pore structure. Asused in this invention, a layer is the resulting deposit of aphospho-alumina precursor solution under the conditions described inthis invention. Due to the irregular surfaces within a porous substrate,multiple deposits of phospho-alumina layers may not form a uniform layerstructure. In some aspects of this invention, each deposited sublayermay occupy the same areas, different areas, or a combination thereof ofthe total open porosity surface area.

In a further aspect of this invention the internal coating of a porousmaterial remains stable up to 800° C. and preferably up to 1000° C. ormore. That is, the internal coating retains compositional integrity andremains crack-free at elevated temperatures. However, in some usesmaterials of this invention may not be used at high temperatures orunder extreme environmental conditions.

An aspect of this invention is coating interior surfaces of an openporous material such that environmental contaminants cannot penetratethrough size constrictions in open channels and preserve interiorporosity and thermal conductivity properties. The number of sublayers ofthe phospho-alumina applied in accordance with this invention may beoptimized to form a suitable porous structure protected fromenvironmental attack but preserving desired physical properties.

Another aspect of this invention is to enable partial or full coating orblockage of very fine (e.g. mesopores) pore channels (<50 nm) byrepetitive coating application that allows for preferential percolationof the chemical solution precursor into the pores. In a preferableaspect of this invention substantially all (and typically more than 98%)pores having mean diameters less than 50 nm (and typically less than 30nm) are coated or blocked by a phosphorus-containing alumina material.Repeated internal coatings of this invention may form a fluid (gas orliquid) diffusion barrier through filling pore or channel constrictionpoints in the porous material. Creating such a constricted, filledbarrier may not be uniform throughout (i.e., may be mostly present nearthe surface of the substrate, e.g., top ˜200 μm) the substrate, but willbe sufficient to form a functional diffusion barrier.

A further aspect of the invention is to use a chemical precursorsolution that preferentially wets the pore walls and yields a densecoating that serves as an effective diffusion barrier along the porewalls and also imparts resistance against sintering or grain growth.Also, the coating provides moisture resistance.

A further additional aspect of this invention is to enable deposition ofpredominantly alumina coatings on pore walls by use of specializedformulations, whereby phosphorous content is added to improve filmforming or wetting characteristics of the coating material.

An additional aspect of the invention is to enable deep penetration ofthe phospho-alumina coating into the substrate such that any erosion orabrasion of substrate surfaces during service or use (such as in turbineshrouds rubbing against blades in gas turbine engines) will noteliminate protection, which will be preserved based on the internalcoating deep within the porous substrate.

Another aspect of the invention is to improve or enhance mechanicalstrengths of substrate materials and their reliability through sealingof internal flaws induced during processing, such as in refractorylinings, advanced ceramic monoliths (e.g., silicon carbide, siliconnitride, mullite, alumina, and combinations thereof), ceramic coatings,and ceramic matrix composites. This invention also may be used in packdiffusion based coatings; monolithic ceramics; metal fiber compositesand mesh; metallic/ceramic foam or scaffold materials; ceramic,metallic, glassy, or organic micropores; microballoons, microbubbles,microporous insulation, including porous silica and alumina/garnet basedgel materials; and ceramic, metallic, glassy/semi-polymeric composites.

A material with open porosity may not necessarily be permeable tofluids. Another aspect of this invention is sealing or decreasedpermeation of fluids (liquids and gases) through porous materials thathave been coated internally according to this invention. Such fluidsinclude molten salts, organic media, jet fuels, water vapor, air,oxygen, carbon monoxide, hydrogen, fuel contaminant vapors, and otherspecies typically found in fuel or coal combustion environments.

An aspect of this invention is an interior-coated open porous materialformed by contacting the porous material with a non-colloidal,low-viscosity liquid coating precursor that allows permeation of theliquid into the open pores and channels of the material. It is believedthat infiltration of the non-colloidal, low-viscosity liquid coatingprecursor forms a liquid film onto the interior walls of the poroussubstrate, which may be 5 nm to 250 nm (preferably 10 to 100 nm) thick.Preferably, such low viscosity precursor solution also has a highconcentration of phosphorus-containing alumina material. Thus, thesolution will yield a greater amount of solids after drying and curing.This high solids yield precursor solution minimizes the number ofrepetitive applications of the solution, which reduces processing costsand potential risks associated with part/component rejection for theseexpensive advanced materials. Such solids yields typically range from atleast 100 g/L (i.e. will produce at least 10 wt % solids based on theprecursor solution) and may range to 300 g/L or above for aluminacoating compositions. Preferably, such a high solids yield solution hasa low viscosity of less than 100 cSt, typically less than 50 cSt, moretypically less than 30 cSt, and more preferably less than 20 cSt.

The pores are in open communication with the exterior surface of thematerial. In this invention, preferably liquid precursor is infiltratedinto the interior pore and channel system. Typically, a method ofapplication of the liquid precursor may be brushing, flowing, dipping,or spraying followed by application of an exterior gas pressure to forma pressure differential between the exterior of the material and theinterior pore and channel system. A preferable method of providing sucha pressure differential is to evacuate the porous material beforeapplying liquid precursor to the surface of the material and thenrepressurizing the material. This will force liquid precursor into theinterior pores and channels to form a thin layer of such liquidprecursor onto the interior surfaces of the porous materials.Alternatively, a porous material to which liquid precursor has beenapplied may be externally pressurized to form a pressure differentialwhich forces the liquid precursor into the interior pore and channelsystem. Typically, a suitable pressure differential is about one bar,although higher or lower exterior pressures may be used. An inert gasmay be used to flush out residual oxygen in a porous material. In avacuum-assisted application method to produce an interior coating ofprecursor, a porous material or an article which has an exterior porousmaterial substrate, typically, may be evacuated to vacuum pressures ofbelow 20 kPa (0.2 bar), typically below 10 kPa, and may be below 1 kPa.Typically, a porous material or article is put under vacuum for a timeperiod sufficient to remove trapped air to the level of the vacuum. Suchtime period may be up to an hour or more, but typically is at least 5 to30 minutes.

In another aspect of an infiltration method of this invention, liquidprecursor is allowed to flow into pore openings of a porous substrate bymaintaining a continuous liquid contact of the surface of the porousmaterial such that liquid precursor migrates into the pore system toform an interior liquid film onto the surface of the interior pores andchannels. In this method a porous material may be immersed or bathed inliquid precursor such that some or all of the pore openings on thesurface remain covered by liquid precursor for a time sufficient forsuch migration. In another method, liquid precursor is sprayedcontinuously or semi-continuously onto a surface of a porous materialsuch that some or all of the pore openings on the surface remain coveredby liquid precursor for a time sufficient for liquid precursor migrationinto the pore structure. Also, continuous contact of liquid precursor toa porous surface may be performed by maintaining such contact with asaturated cloth, sponge, or similar material for a sufficient time forliquid migration into the pore structure. Typically, immersion orcontinuous contact of the surface with liquid precursor of at leastabout 5 minutes, typically at least 10-15 minutes, and may be at 20minutes or longer as needed to permit such migration. Such continuouscontact is distinct from mere brushing, flowing, dipping, or spraying ofa liquid onto a surface in which there is insufficient time to permitliquid migration into the interior pore structure.

Layers of phosphorus-enriched aluminas or phospho-aluminas used in thisinvention may be made typically by forming a sol-gel containing oxidesof aluminum to which oxides of phosphorus are incorporated, which iscured by heating to a temperature sufficient to form a phospho-alumina.These coating precursor materials typically are formed in a non-aqueousliquid such as an alcohol and not under strong acid conditions (pH>2). Atypical low viscosity, non-colloidal (i.e., clear, non-cloudy) solutionof precursor material useful in this invention is formed by combining analuminum salt in alcoholic solution with an alcoholic solution of aphosphate ester or precursor of a phosphate ester. A typically suitablealcohol solvent is a C₁-C₄ monohydroxy alcohol including methanol,ethanol, n-propanol, iso-propanol, n-butanol, 2-butanol, and t-butanoland combinations thereof. Methanol, ethanol, or combinations thereof arepreferred. In a typical procedure, alcohol solutions of an aluminum salt(such as aluminum nitrate or aluminum acetate) and a phosphorus oxidesuch as phosphorus pentoxide (P₂O₅) or phosphorus oxide ester arecombined in aluminum to phosphorus atomic ratios suitable for creating adesired layer of material. It is believed that an alcohol solution of aphosphorus pentoxide results in an ester of a phosphorus oxide orphosphate. Preferably, the liquid precursor is a non-colloidal solutionof the phospho-alumina precursor in an alcohol solvent. Preferably,concentration of a phospho-alumina precursor liquid is at least about 1molar (M) and typically is 1.5 M or above and may range up to 3 M orabove wherein molarity/concentration is the number of moles of Al perliter of solution. A typical phospho-alumina precursor is 1 to 2.75 Mand preferably 1.5 to 2.5 M. Use of lower phospho-alumina precursorconcentrations (e.g., >0.3 M) typically requires additional coatings,which may be inefficient. Preferably, the coating materials are halidefree. Examples of phosphorus-alumina coating systems are described inU.S. Pat. Nos. 6,036,762, 6,461,415, 7,311,944, 7,678,465, 7,682,700,all incorporated by reference herein.

A typically useful phospho-alumina precursor coating solution has analuminum to phosphorus atomic ratio at least 0.5:1 and may range up to20:1, or above. Typical phospho-alumina precursors have analuminum-to-phosphorus atomic ratio at least 1:1 and usually at least2:1, and preferably at least 4:1. Such typical precursors may have analuminum-to-phosphorus atomic ratio up to 15:1 and preferably up to10:1.

The principal structural components of the phospho-alumina precursorsolutions useful for the layers used in this invention appear to becomplexes that contain Al—O—Al linkages. From analysis of ²⁷Al and ³¹PNMR data, the internal structure of the precursor materials is such that[PO₄] groups appear to be linked to [AlO₄] groups, which in turn arelinked to [AlO₆] groups. Thus, these materials contain tetrahedralaluminum coordination together with “distorted” octahedral aluminum, theintensity of which distortion increases with increases in excessaluminum content. This is unlike exclusive tetrahedral coordination foraluminum observed in crystalline polymorphs of AlPO₄.

Typical phospho-aluminas useful in this invention may contain Al—O—Allinkages and may contain [PO₄] tetrahedra groups linked to [AlO₄]tetrahedra groups, which in turn are linked to [AlO₆] octahedral groups.These phospho-aluminas therefore are distinct from aluminophosphatepolymorphs that exist in tetrahedral coordination.

Phospho-aluminas, or otherwise described as phosphorus-enriched orphosphorus-containing aluminas, in which the Al/P atomic ratio is lessthan about 12, typically less than about 10, usually less than about 8,useful in this invention typically are substantially amorphous and suchamorphous character may be determined by X-ray diffraction (XRD)spectra. A substantially amorphous material does not exhibit specificXRD peaks, which can be attributed to lattice parameters of acrystalline structure. Typically, phosphorus-enriched aluminas in whichthe Al/P atomic ratio is more than 6 may convert partially or fully tocrystalline alumina polymorphs upon high temperature exposures over longterm. Despite this transformation, in the case of CMAS attack, thepartially or fully transformed alumina will remain effective as asacrificial layer to arrest CMAS flow and further TBC degradation.

A liquid coating precursor suitable for use in this invention shouldhave a viscosity sufficiently low to permit permeation (i.e., nearcomplete infiltration) of the liquid into the interior pore and channelsystem under the pressure differential conditions used. Typically, theliquid precursor is an alcohol solution, such as methanol or ethanol,and has a kinematic viscosity of below 150 centistokes (cSt), usuallybelow 100 CSt, typically below 60 cSt preferably below 50 cSt, below 30cSt or below 20 cSt. A preferable solution contains ethanol, methanol,or combination thereof and has a viscosity below 35 cSt. The lowviscosity liquid should have low surface tension to facilitateinfiltration into fine channels and pores. Typically, viscosity may bemeasured according to ASTM D446.

In accordance with this invention, one or more layers of a modifiedalumina material are deposited upon the side surfaces of interiorchannels of a porous substrate. Preferably, the side surfaces of thechannels are coated without blocking or filling channels havingdiameters of more than 30-50 nm. Thus, most of the channels are open andare in open communication with the surface. This retains properties ofthe porous substrate such as thermal barrier protection and stressfracture resistance.

Typically, each sublayer is 1 to 50 nm thick, such that as layers arebuilt up, small mesopores (e.g. 30 nm or less in average diameter)channels may be completely filled may be blocked at narrow channelconstrictions. However, the remaining coated channels have a pore volumesufficient to maintain beneficial properties of the porous material.Depending on the application an internal coating layer of this inventiontypically is at least 5 nm (usually at least 10 nm or at least 20 nm)and may be up to 500 nm, preferably up to 300 nm, and typically up to100 nm thick. A typical thickness range for an internal coating layer(including sublayers) is 10 to 300 nm.

Application of multiple sublayers of phospho-alumina in accordance withthis invention is preferable to form an advantageous conformal interiorprotective coating layer within a porous material. Typically, at leasttwo coating applications are used and many times up to ten to twentysublayers or more of phospho-alumina composition may be used. Typicaluseful applications use two to five sublayers of phospho-aluminacomposition. In order to form a sublayer, the open porous material istreated as described by applying a low viscosity liquid precursor to thematerial exterior followed by a pressure differential treatment. Afterthe liquid precursor has formed an interior surface coating within theporous material, the material is dried and cured to form aphospho-alumina sublayer. This process may be repeated to form multiplesublayers of phospho-alumina composition. Although multiple sublayersmay improve protective properties, application of multiple sublayerswill fill the open pore and channel system, which may affect somephysical or mechanical properties of the material; however, typicallythe total weight gain from coating application within the pores is lessthan 10 wt. % and typically is less than 5 wt. % and may be less than 2wt. %. Typically, two to five sublayers will be sufficient to form aprotective layer without significantly affecting desired mechanicalproperties. Sublayers of phospho-alumina composition useful in thisinvention may have different Al/P ratios.

A particular advantage of multiple layer application is to allowpercolation of coating liquid into extremely fine pore channels.Although two repetitive cycles of coating application does not appear toinfiltrate very fine channels (which typically are less than 50 nm andmay be less than 30 nm), five layers of coating appear to fill the veryfine channels completely as indicated by transmission electronmicroscopy for the columnar pore channels in a TBC coating. Withoutwishing to be bound by any theory, it is postulated that percolation oflow-viscosity liquid precursor coating solution upon multipleapplications of coating solution restricts segments in larger porechannels until sufficient internal pressure is obtained to force suchcoating liquid into the very fine channels. This effect may be based onthe fact that pore channel sizes are not uniform in the porous body. Forexample, a pore channel may have a maximum 2-micrometer diameter in manysegments of the channel, but may contain constrictions of less than 100nm in certain locations. This is typical, for example, in polymerinfiltrate pyrolysis (PIP) used in fabrication of ceramic matrixcomposites. Upon multiple infiltrations of the coating solution, the“constriction” points within a pore channel (such as in the 100 nmconstrictions in the example above) are further reduced, whichfundamentally alters the percolation of the precursor coating solutionduring a subsequent infiltration step. The presence of suchconstrictions during a subsequent infiltration results in increasedinternal pressure at the entrance of such fine channels, which directsinfiltration of the coating solution into the fine channels. This effectparticularly is useful if the very fine channels are primarilyresponsible for the property or properties of interest of the materialin service, such as hot corrosion degradation, because of the highersurface area per volume of such very fine channels. This type ofdegradation occurs in atmospheric plasma sprayed (APS) TBC coatings.However, the invention is applicable to ceramic coatings produced byother techniques, including but not limited to, vacuum plasma spray(VPS), High Velocity Oxy Fuel (HVOF), and electron beam physical vapordeposition (EBPVD).

Coverage of internal structures of porous materials withphosphorus-containing alumina described in this invention typically maybe observed using scanning electron microscopy (SEM) or transmissionelectron microscopy (TEM) with representative samples of such materials.

An aspect of this invention is a porous substrate in which macroporesare internally coated, and mesopores are substantially (e.g. >50%)filled or blocked, with phosphorus-containing alumina described in thisinvention. In some aspects, constrictions within the pore and channelsystem in the porous substrate are blocked, which transforms an openpore system into a partially closed pore system. Because much of thepore structure remains, the density of the porous substrate may notchange significantly (<10%, typically <5%, preferably <1%) afterapplication of internal coating. This may be observed by measuringweight gain (or density) of a porous substrate after internal coating inaccordance with this invention.

The application of a phosphorus-containing alumina coating of thisinvention in some applications may be repeated multiple times, until themesopore porosity of the TBC system is reduced to less than 5%,representing more than 20 applications of the phospho-alumina coating.The goal of applying the coating using this approach of combining athermally stable amorphous phospho-alumina coating material made from alow viscosity precursor and a infiltration coating process targeted atspecific internal substrate pore sizes is to use an infiltration processrepeatedly to apply the coating material in a stepwise fashion ofsuccessive layers until the substrate's fine pore channels/structuresare lined with the coating material or nearly filled with the coatingmaterial, without significant impact to the inherent strain tolerance ordensity of the substrate, with curing or setting of the coating donebetween each layer for coating stability purposes.

In some applications of this invention a porous material (e.g., fusedsilica or silicon nitride) is treated multiple times such that pore orchannel constrictions within the substrate are effectively blocked suchthat fluid (liquid or gas) permeation through the substrate is blocked,although the substrate material may continue to have a degree of openporosity. In such applications, the amount of remaining open porosityafter treatment according to this invention typically may be at least30%, 40% or 50% of an untreated substrate.

In some typical uses of this invention, a porous material is bonded toanother substrate material to form a composite or laminated structure.The exterior porous material may act as a protective structure to theunderlying substrate. Typical underlying substrates are metals andbonded porous materials are oxides. A bondcoat may be applied between ametal substrate and a porous substrate. In this invention, theinternally coated porous substrate material will protect the underlyingmaterial from environmental attack such as oxidation. Thermally grownoxide (TGO), present underneath TBC coatings, formed from oxidation ofunderlying bondcoat alloys can induce interfacial stresses that leads todelamination or TBC failure. This is further exasperated by interfaceroughening or rumpling of TGO as it grows. Reduction in TGO scale growthor scale of roughening would extend the life of a turbine componentcoated with such a TBC. Thus, in general, oxidation of metal or metallicalloy having a ceramic coating can be subject to such stresses andsubsequent cracking/failure. Internal coatings, such as described inthis invention, can reduce the oxidation rate and thereby provideenhanced protection. The exterior porous material may be of any suitablethickness, which provides an adequate protective layer to the underlyingsubstrate. Typical thermal barrier coatings are 0.1 to 2 millimeters,although the porous material may be thinner or thicker depending uponspecific use.

The composition of the advanced high performance materials useful inapplications of this invention is chosen based on the functional needrequired of the material, but is most strongly influenced by theenvironmental conditions described above and the ability of the materialto withstand the environment. For example, carbon is an excellentthermal material for heat management applications, because carbon canconduct large quantities of heat for removal in large thermal loadapplications. However, because carbon is susceptible to extremely rapidoxidation, carbon is not useful in oxidizing environments and cannot beused in pure form. Other materials substituted for carbon in hightemperature oxidizing conditions typically are heavier, not as goodthermal conductors, and not as easy to form or machine as pure carbon(such as graphite). Thus, additional substituted material is required tomeet the functional effectiveness of carbon, which impacts cost andweight restrictions. Use of an interior coating as described in thisinvention preserves the physical and mechanical properties of a material(such as thermal conductivity) without significant impact on weight.

A preferable interior layer (which may have more than one sublayers) ofphospho-alumina within a porous material in this invention conforms tothe interior surfaces of the material. The phospho-alumina is continuousthroughout the interior surface and is substantially crack free to theextent that the underlying substrate material (such as a metal) is notexposed to harsh environmental conditions during use. Thephospho-alumina layer has low oxygen permeability and thus functions asan oxidation protection barrier. Further, this layer is a barrier tochemical reactions that cause corrosion of the porous material andunderlying substrates.

In a method of this invention, a sol-gel composition containing oxidesof aluminum and phosphorus is dried typically at a mild elevatedtemperature (usually 60 to 200° C.) or reduced pressure to removevolatile organics, and then cured by heating to at least the meltingtemperature of the resulting mixture for a time sufficient to form aflowable fluid. Such fluid state may form a film or layer within theinternal pores of a substrate having a substantially uniform (typically<10% variation) phospho-alumina composition throughout the sublayer andwhich layers conform to the internal surfaces of the pores. Typicalcuring temperatures useful in this invention are at least 350° C.,usually above 400° C. and preferably above 500° C. and may range up to900° C. or more. A suitable curing temperature is below thedisintegration temperature of the materials. A suitable curing time maybe a few minutes and typically may range from 5 minutes to up to 3 hoursor more. Drying and curing may be conducted in one continuous step.

In another aspect of this invention, an aluminum-phosphorus precursorsolution of this invention may be applied to a suitable substrate asdescribed and dried, but not cured. Such a coated substrate may beincorporated into a component, which may be subject to high temperatureconditions during service. If such conditions become higher than themelting temperature of the uncured coating, the coating will betransformed into a phospho-alumina as described in this invention, andwill become a protective barrier coating to the substrate.

Illustrative examples of articles having porous surfaces, which may usethis invention include, but are not limited to, thermal barrier coatingsused on aircraft engine and power generation turbine blades,refractories used in applications such as metal processing; ceramicmatrix composites such as based on carbon-carbon, carbon-siliconcarbide, and silicon carbide-silicon-carbide composites; graphite;ceramic materials; and the like.

Aspects of the invention are illustrated but not limited by thefollowing examples.

Precursor Solution A

A precursor solution was prepared by adding aluminum nitrate nonahydrate(GFS Chemicals, Powell, Ohio) to anhydrous ethanol (˜375 g/liter). In aseparate container, phosphorus pentoxide (Sigma Aldrich, St. Louis, Mo.)was dissolved in anhydrous ethanol (˜284 g/l) and then the two solutionswere combined and stirred under reflux conditions for 16 hours. Thesolution was concentrated in a rotary evaporator at 65° C. The resulting116 g/liter phospho-alumina precursor solution had an aluminum tophosphorus atomic ratio of 10 to 1.

Precursor Solution B

A precursor solution was prepared in a manner similar to precursorsolution A by adding aluminum nitrate nonahydrate to methanol (˜375g/liter). In a separate container, phosphorus pentoxide was dissolved inmethanol (˜284 g/l) and then the two solutions were combined and stirredunder reflux conditions for 40 hours. The solution was concentrated on arotary evaporator at 65° C. The resulting 181.9 g/L phospho-aluminaprecursor solution had an aluminum to phosphorus atomic ratio of 1.5 to1.

Example 1

A thermal barrier coating (TBC) on a substrate consisting of anickel-based superalloy coupon substrate with an overlying NiCrAIYbondcoat layer and a top plasma spray coating of yttria-stabilizedzirconia (Praxair Surface Technology) was coated with precursor solutionA. The TBC zirconia topcoat porosity was measured by Archimedes' methodto be in the range of 10%-20% before applying the precursor solution. Acoating on the external surface and internal open pore wall surfaces wasapplied using vacuum assisted infiltration of precursor solution A. TheTBC substrate first was cleaned by immersing in acetone and alcoholwhile ultrasonicating for at least 10 minutes each, followed by dryingin air at 120° C. for 30 minutes. The substrate was placed in a glassbeaker and placed in a vacuum chamber, which was evacuated to a pressurein the range of 1-20 torr (0.1-3 kPa) to, with a dwell time under fullvacuum of 5 minutes. The coating precursor solution was introduced intothe vacuum chamber and directed into the beaker holding the TBCsubstrate through a delivery line. The TBC sample then was allowed tosoak while completely submerged under the level of the coating precursorliquid for about 30 minutes, while still under (passive) vacuum, suchthat the system pressure was allowed to rise naturally as the alcoholicprecursor solution evaporated. The TBC substrate was removed from thevacuum chamber and the solution container, and dried at 120° C. for 30minutes. Subsequently, the substrate sample was directly inserted into afurnace set at 500° C. in air and allowed to dwell for 45 minutes tofacilitate the curing of the aluminum phosphate coating.

Example 2

Coated TBC samples of Example 1 were characterized using electronmicroscopic methods of the TBC substrates prepared by fracturing the TBCalong a plane that was oblique to the TBC topcoat thickness, whichallowed direct observation of the aluminum phosphate coating within theTBC porosity. Scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM) were used to confirm that the coating waswell-adhered to the internal TBC pore surfaces and to show the presence,continuous coverage, high density, amorphous structure, and hermeticnature of the phospho-alumina coating. Pore sizes in the YSZ TBC systemsubstrate were in three distinct ranges: 4 nm-20 nm mean pore diameter(as defined by TEM instrument resolution), 20 nm to 100 nm, and 100 nmto 2 mm. The phospho-alumina coating material was confirmedsubstantially to be present in the pores in size ranges of 4 nm to 100nm mean pore diameter and 100 nm to 1000 nm.

Example 3

Phospho-alumina coated TBC samples similar to those of Example 1(infiltrated 2 times (2×) and 5 times (5×)) were annealed at 1300° C.for 1 hour in air in a tube furnace under atmospheric conditions with a30 mg Calcium Magnesium Aluminum Silicate (CMAS) pellet sitting on topof each sample. The CMAS was prepared as a powder by combining the oxidepowders of calcium (35 weight %), magnesium (10 weight %), aluminum (7weight %), and silicon (48 weight %) that was pressed into a pellet. Anuncoated TBC sample with a 30 mg CMAS pellet on top also was includedfor exposure as a control.

After annealing, a significant increase in resistance to attack andcorrosion of the TBC by the CMAS material was noted for thephospho-alumina coated samples, as measured by the lack of infiltrationof the CMAS into the TBC porosity. In addition, spallation of thezirconia topcoat after the CMAS exposure for the uncoated TBC, alongwith general densification of the microstructure of the uncoated TBC ascompared to the post-exposure status of the coated samples of Example 3,indicated an increased degree of CMAS attack for the uncoated TBC samplecompared to the coated samples. Thus, the mitigation of CMAS attack forthe coated samples was a result of localized presence of “alumina” oninternal pore walls.

While both the 2× infiltration and 5× infiltration phospho-aluminacoated samples showed increased resistance to CMAS attack compared tothe uncoated TBC sample, the performance of the 5× infiltration samplewas superior based on the depth and degree of molten CMAS attack. Thismay be due to the local protection provided by the infiltration of thefinest TBC porosity (4-100 nm) that occurs for the 5× infiltrationsample but not the 2× infiltration sample.

Example 4

Phospho-alumina coated TBC samples with two different YSZ architectureswere prepared according to Example 1, but with 1× and 2× coating processruns. Thermal exposures were performed with the samples in air from roomtemperature to 1121° C. in a cyclic fashion in a vertically aligned tubefurnace with a pulley assembly, with dwell times of 15 minutes and 45minutes at each temperature, respectively. The primary purpose of thisexperiment was to demonstrate retention of strain tolerance of the TBCcoating. The thermal exposures were designed to induce mechanicalfailure within the coating layers of the TBC samples; the mechanism andtime to failure should be a direct indicator of the strain tolerance ofthe TBC. Several uncoated TBC samples of identical architecture as thecoated samples were included in the thermal exposure for reference.Thermally induced mechanical failure of the TBC ceramic topcoat (i.e.,loss of adhesion) was observed to occur at 139 thermal cycles for theuncoated and an average of 137 thermal cycles for the phospho-aluminacoated samples for the first YSZ architectural variant samples, and nofailure was observed after an average of 777 thermal cycles for bothuncoated and phospho-alumina coated samples for the second YSZarchitectural variant. The predominant failure mechanism for the firstarchitectural variant sample was related to ceramic topcoat stiffeningfor the uncoated sample, which was in contrast to the phospho-aluminacoated samples failure mechanism being related to de-adherence of thethermally grown oxide layer underlying the ceramic topcoat layer. Thesedata show that the application of the coating does not reduce the straintolerance and potentially improves TBC performance due to enhancedsintering resistance.

Example 5

Four samples of ceramic matrix composites (CMC) substrates consisting ofa silicon carbonitride matrix with silicon carbide fibers were coated:Substrate 1 was coated with precursor solution B (181.9 g/L, 14.2 cSt);Substrate 2 was processed with a similar precursor solution (103 g/L, 15cSt; Al:P 4:1) with a lower yield and similar viscosity to precursorsolution B; Substrate 3 was coated with a similar, ethanolbased-precursor solution (151 g/L, 34 cSt; Al:P 4:1) with a lower yieldand higher viscosity compared to precursor solution B; and Substrate 4was processed with a similar, ethanol based-precursor solution (188 g/L,59.75 cSt; Al:P 4:1) with a higher yield and higher viscosity comparedto precursor solution B. The CMC open porosity was measured prior tocoating by Archimedes' method to be approximately 5% before applying theprecursor solution. The coating was applied to the external and internalpore wall surfaces of each CMC sample using vacuum assistedinfiltration. Each substrate was cleaned by immersion separately underacetone, methanol, and ethanol while ultrasonicating for 10 minutes eachfollowed by a drying step at 120° C. in air for 30 minutes. Thesubstrate was placed in a glass, graduated cylinder with a vacuum fittedcap with two ports for vacuum and the introduction of the precursorsolution. A vacuum pump was used to evacuate the cylinder to a pressureof 5 to 20 torr. The graduated cylinder was filled with the precursorsolution through the second port such the substrate was completelysubmerged. The CMC substrate was allowed to sit submerged in theprecursor solution while under vacuum for approximately 1 minute. Afterinfiltration, the vacuum was released and sample was removed from thegraduated cylinder and dried in air at 120° C. for 30 minutes. Followingthe drying, the sample was placed in a furnace and ramped from roomtemperature to 550° C. at a rate of 10° C./minute. The sample was heldat 550° C. for 1.5 hours to facilitate the curing of the aluminumphosphate coating and then cooled to room temperature at 10° C./minute.This process was repeated a total of 5 times for each substrate.

The coated CMC samples were subjected to environmental testing involvinga 15-minute submersion in deionized water, followed by 24 hours in ahumidity chamber at 90% relative humidity and 32° C. The weight of eachsample was measured after removal from the humidity chamber to determinethe amount of moisture absorbed. It is believed that limiting the accessof moisture to the internal porosity of the material as indicated by thelimited weight gain mitigates degradation of the porous substrate. Thewater submersion and humidity chamber steps were followed by 8 hours inan oven at about 800° C. with a heating and cooling rate of 10°C./minute. This 3-step exposure was repeated 8-times and an uncoated CMCsample also was included as a control. Following completion of the8-cycle exposure, the ultimate tensile strengths of the samples weremeasured according to ASTM C1275. The percent decrease in tensilestrength of the samples following the accelerated environmental exposureas compared to the uncoated, as-received material are shown along withthe moisture weight gain in Table 1. The data in Table 1 indicate thatthe coating is mitigating the CMC degradation.

TABLE 1 Precursor Precursor Weight Tensile Solution Solution GainStrength Sample¹ Yield (g/L) Viscosity (cSt) (%)² Decrease (%)³ UncoatedCMC 1.3 59 Substrate 1 181.9 14.2 0.3 3 Substrate 2 103 15.0 0.5 22Substrate 3 151 34.0 0.7 56 Substrate 4 188 59.8 1.0 50 ¹After 8-cycleenvironmental exposure ²Weight gain after liquid water submersion andhumidity chamber steps ³Decrease in tensile strength versus uncoated“as-received” sample

Example 6

Phospho-alumina coated TBC samples were prepared according to Example 1,but with two independent 1× coating process runs with different Al/Pratios. The coated samples together with two uncoated samples ofidentical architecture were subjected to a thermal exposure test byheating in air at 1100° C. for a cumulative total of 338 hours at thattemperature. Periodically, during the thermal exposure, the samples werecooled at a rate slower than 10° C./min until they reached roomtemperature and weighed and, thereafter, reheated to 1100 C. at a rateof 10° C./min. Weights were measured at 1, 3, 6, 8, 13, and 17 daysduring the thermal exposure test. The thermal exposure test was designedto induce mechanical failure within the coating layers of the TBCsamples by oxidation of the metallic layer underneath the ceramictopcoat. Oxide scale thickening and lengthening rates are a directindicator of the oxidation resistance of the TBC system. Oxidationinduced mechanical failure of the TBC ceramic topcoat (loss of adhesion)was observed to occur after 266 hours for one of the uncoated samples,while the other uncoated and the phospho-alumina coated samples remainedintact for the remainder of the 338 hour exposure. Oxide scale growthrate was measured as a function of weight gain per unit surface area perunit time and referenced to the original sample weight per surface areato yield a percent weight change over time. Results of the thermalexposure tests are shown in Table 2. These data show that application ofthe 1× phospho-alumina coating with an Al/P ratio of 2 substantiallydecreased the overall oxide scale formation rate of the metal substratebelow the TBC topcoat. This indicates that the coating has penetratedthe porous TBC and created a barrier against oxygen on or near the metalsurface. Additional treatments (up to 5×) may result in furtherenhancement of protection against oxidation or environmental protectionagainst other gaseous species.

TABLE 2 Weight Rate of Weight Coating Change Change Sample Al/P (%)(%/hour × 10⁴) Example 6A 6 27 4.8 Example 6B 2 21 2.9 Uncoated Run A — (1) (1)   Uncoated Run B — 34 5.6 ¹Spalled after 266 hours

Precursor Solution C

A precursor solution was prepared in a manner similar to precursorsolution A by adding aluminum nitrate nonahydrate to anhydrous ethanol(˜375 g/liter). In a separate container, phosphorus pentoxide wasdissolved in anhydrous ethanol (˜284 g/liter) in an inert atmosphereglove box and then the two solutions were combined and stirred underreflux conditions for 16 hours. The solution was concentrated on arotary evaporator at 65° C. The resulting 173 g/L phospho-aluminaprecursor solution had an aluminum to phosphorus atomic ratio of 2 to 1.

Precursor Solution D

A precursor solution was prepared in a manner similar to precursorsolution A by adding aluminum nitrate nonahydrate to anhydrous ethanol(˜375 g/liter). In a separate container, phosphorus pentoxide wasdissolved in anhydrous ethanol (˜284 g/liter) in an inert atmosphereglove box and then the two solutions were combined and stirred underreflux conditions for 16 hours. The solution was concentrated on arotary evaporator at 65° C. The resulting 4.0 molar (346 g/L)phospho-alumina precursor solution had an aluminum to phosphorus atomicratio of 2 to 1.

Precursor Solution E

A precursor solution was prepared in a manner similar to precursorsolution A by adding aluminum nitrate nonahydrate to anhydrous ethanol(˜375 g/liter). In a separate container, phosphorus pentoxide wasdissolved in anhydrous ethanol (˜284 g/liter) in an inert atmosphereglove box and then the two solutions were combined and stirred underreflux conditions for 40 hours. The solution was concentrated on arotary evaporator at 65° C. The resulting 1.0 molar (86.5 g/L)phospho-alumina precursor solution had an aluminum to phosphorus atomicratio of 2 to 1.

Precursor Solution F

A precursor solution was prepared in a manner similar to precursorsolution A by adding aluminum nitrate nonahydrate to anhydrous ethanol(˜374 g/liter). In a separate container, phosphorus pentoxide wasdissolved in anhydrous ethanol (˜284 g/liter) in an inert atmosphereglove box and then the two solutions were combined and stirred underreflux conditions for 40 hours. The solution was concentrated on arotary evaporator at 65° C. The resulting 2.5 molar (246 g/L)phospho-alumina precursor solution had an aluminum to phosphorus atomicratio of 1.5 to 1.

Precursor Solution G

A precursor solution was prepared in a manner similar to precursorsolution A by adding aluminum nitrate nonahydrate to anhydrous ethanol(˜375 g/liter). In a separate container, phosphorus pentoxide wasdissolved in anhydrous ethanol (˜284 g/liter) and then the two solutionswere combined and stirred under reflux conditions for 40 hours. Thesolution was concentrated on a rotary evaporator at 65° C. The resulting300 g/L phospho-alumina precursor solution had an aluminum to phosphorusatomic ratio of 1 to 1 and a viscosity of 59 cSt.

Example 7

A coupon of slip cast fused silica (SCFS) was coated with precursorsolution C. The SCFS open porosity was measured by He gas pycnometry tobe in the range of 14%-16% before applying the precursor solution. Acoating on the external surface and internal open pore wall surfaces wasapplied using vacuum assisted infiltration of the precursor solution C.The SCFS substrate first was cleaned by immersing in acetone and alcoholwhile ultrasonicating for at least 10 minutes each, followed by dryingin air at 120° C. for 30 minutes. The substrate was placed in a glassbeaker and placed in a vacuum chamber, which was evacuated to a pressurein the range of 0.1-20 torr (0.01-3 kPa) to, with a dwell time underfull vacuum of 5 minutes. Subsequently, the coating precursor solution Cwas introduced into the vacuum chamber and directed into the beakerholding the SCFS substrate through a delivery line. The SCFS sample thenwas allowed to soak while completely submerged under the level of thecoating precursor liquid for about 60 minutes, while still under(passive) vacuum, such that the system pressure was allowed to risenaturally while some alcohol from the precursor solution evaporated. TheSCFS substrate was removed from the vacuum chamber and the solutioncontainer, and dried at 60° C. for 30 minutes and then 120° C. for 30minutes. Subsequently, the substrate sample was inserted into a furnaceand ramped to 600° C. and a rate of 10° C./min in air and allowed todwell for 120 minutes to facilitate the curing of the aluminum phosphatecoating. This procedure was repeated a total of four times with thefirst three with precursor solution C and finally using precursorsolution D. After completion of four cycles, open porosity was measuredby He pycnometry to be 7.2%, i.e., a 50% decrease in open porositycompared to the original SCFS sample (measured from all externalsurfaces).

Example 8

A coupon of coated SCFC, made according to Example 7, was tested forflexural mechanical strength in accordance with ASTM D790. The uncoatedand coated material exhibited average flexure strengths of 58 and 67mPa, respectively. An additional coupon was made according to Example 7and was tested for He gas permeability using a Veeco MS-170 He leakdetector (Oneida Research Services, Whitesboro, N.Y.). The coated anduncoated material exhibited He leak rates of 3×10⁻⁸ and 2.0×10⁻⁸atm·cc/sec, respectively.

Example 9

A coupon of slip cast fused silica (SCFS) was coated with precursorsolution G. The SCFS sample was measured by calipers and an analyticalbalance to have a bulk density of 1.95 g/cc. The SCFS was additionallymeasured by He gas pycnometry and reported in Table 3 as open pores andclosed pores (calculated by the bulk density). A coating on the externalsurface and internal open pore wall surfaces was applied using vacuumassisted infiltration of precursor solution G. The SCFS substrate firstwas cleaned by immersing in alcohol while ultrasonicating for at least10 minutes each, followed by drying in air at 160° C. for 10 minutes andheating at 800° C. for 60 minutes. The substrate was dried at 160° C.and placed in a glass beaker and placed in a vacuum chamber, which wasevacuated to a pressure in the range of 0.1-20 torr (0.01-3 kPa) to,with a dwell time under full vacuum of 10 minutes. The coating precursorsolution was introduced into the vacuum chamber and directed into thebeaker holding the SCFS substrate through a delivery line. The SCFSsample then was allowed to soak while completely submerged under thelevel of the coating precursor liquid for about 60 minutes, while stillunder vacuum, such that the system pressure was allowed to risenaturally while some of the alcohol from the precursor solutionevaporated. The SCFS substrate was removed from the vacuum chamber andthe solution container and patted dry with a Kimwipe™, and dried at 60°C. for 10 minutes and then 160° C. for 30 minutes. Subsequently, thesubstrate sample was inserted into a furnace and ramped to 800° C. and arate of 10° C./min in air and allowed to dwell for 60 minutes tofacilitate the curing of the aluminum phosphate coating. This procedurewas repeated a total of five times with precursor solution G. Afterdeposition of each layer, the SCFS porosity (vol. %) was measured byhelium gas pycnometry and reported in Table 3 as open pores and closedpores (calculated by the bulk and powder density), together with thepercent weight gain per layer. A leak rate for the material wasdetermined by fastening the coupon to a vacuum chuck with crystal bondand attaching to a vacuum line equipped with a pressure gauge(Baratron™). The system was evacuated for 60 minutes and the system wassealed off with a valve such that a pressure of <5 torr was made betweenthe sample and the pressure gauge. A leak/diffusion rate of air throughthe coupon was recorded over ten minutes. The leak rate of unsealed SCFSand compared to the sample above was 31.2 torr/min vs. 0.03 torr/min,respectively.

TABLE 3 Layer Open pores Closed pores % weight gain Uncoated 15% 13% N/A1 layer 17% 11% 1.58% 2 layers 15% 13% 0.08% 3 Layers 15% 13% 0.07% 4Layers 13% 15% 0.08% 5 Layers  0% 28% 0.01%

Example 10

A coupon of reaction bonded silicon nitride (RBSN) was coated withprecursor solution F. The RBSN open porosity was measured by Archimedes'method before applying the precursor solution to be 23%. A coating onthe external surface and internal open pore wall surfaces was appliedusing vacuum assisted infiltration of the precursor solution C. The RBSNsubstrate first was cleaned by immersing in acetone and alcohol whileultrasonicating for at least 10 minutes each, followed by drying in airat 120° C. for 30 minutes. The substrate was placed in a glass beakerand placed in a vacuum chamber, which was evacuated to a pressure in therange of 0.1-20 torr (0.01-3 kPa) with a dwell time under full vacuum of5 minutes. Subsequently, the coating precursor solution C was introducedinto the vacuum chamber and directed into the beaker holding the RBSNsubstrate through a delivery line. The RBSN sample then was allowed tosoak while completely submerged under the level of the coating precursorliquid for about 10 minutes, while still under (passive) vacuum, suchthat the system pressure was allowed to rise naturally while somealcohol from the precursor solution evaporated. The RBSN substrate wasremoved from the vacuum chamber and the solution container, and dried at120° C. for 10 minutes. Subsequently, the substrate sample was insertedinto a furnace and ramped to 500° C. and a rate of 10° C./min in air andallowed to dwell for 60 minutes to facilitate the curing of the aluminumphosphate coating. This procedure was repeated a total of five times.After completion of the five cycles the open porosity was measured byArchimedes' method to be 1%, thus a decrease in open porosity comparedto the original RSBN sample.

Example 11

A coupon of coated RBSN, made according to Example 10, was tested forflexural mechanical strength in accordance with ASTM D790. The uncoatedand coated material exhibited average flexure strengths of 103 and 128mPa, respectively.

Precursor Solution H

A precursor solution was prepared in a manner similar to precursorsolution A by adding aluminum nitrate nonahydrate to anhydrous ethanol(˜375 g/liter). In a separate container, phosphorus pentoxide wasdissolved in anhydrous ethanol (˜284 g/liter) and then the two solutionswere combined and stirred under reflux conditions for 40 hours. Thesolution was concentrated on a rotary evaporator at 65° C. The resulting100 g/liter phospho-alumina precursor solution had an aluminum tophosphorus atomic ratio of 1.5 to 1 and a viscosity of 5.3 cSt.

Example 12

Samples of SCFS with bulk volumes of ˜0.6-0.9 cc were chemically cleanedin ethanol in an ultrasonic bath and then further cleaned by heattreating at 800 C. for 1 hour in a furnace. After the samples were driedin an oven at 160° C. for 10 minutes, the samples were immersed inPrecursor Solution H for various times. The samples were removed fromthe solution and patted dry with a Kimwipe™ to remove excess solution.The samples were dried in an oven at 60° C. for 10 minutes and 160° C.for 10 minutes. Subsequently, the substrate sample was inserted into afurnace and ramped to 800° C. and a rate of 10° C./min in air andallowed to dwell for 60 minutes to facilitate the curing of the aluminumphosphate coating. The samples were weighed on an analytical balance andthe percent weight gain was calculated. Table 4 displays the percentweight gains calculated for cured aluminum phosphate layer exhibitingthe time dependence of the immersion process. The weight gains for 5 to60 minutes do not show a statistical difference.

TABLE 4 Immersion Time % weight gain  1 minute 0.26%  5 minutes 0.45% 10minutes 0.51% 20 minutes 0.47% 60 minutes 0.46%

Example 13

Two samples of SCFS were chemically cleaned in ethanol in an ultrasonicbath and then further cleaned by heat-treating the samples at 800° C.for 1 hour in a furnace. The samples were dried in an oven at 160° C.for 10 minutes. One sample was immersed in Precursor Solution H for 1minute and the other sample was immersed for 60 minutes. The sampleswere removed from the solution and patted dry with a Kimwipe™ to removeexcess solution. The samples were dried in an oven at 60° C. for 10minutes and 160° C. for 10 minutes. Subsequently, the substrate samplewas inserted into a furnace and ramped to 800° C. and a rate of 10°C./min in air and allowed to dwell for 60 minutes to facilitate thecuring of the aluminum phosphate coating. This procedure was repeatedsix times. A leak rate for the material was determined by fastening thecoupon to a vacuum chuck and crystal bond and attaching to a vacuum lineequipped with a Baratron™ pressure gauge. The system was evacuated for60 minutes and the system was sealed off with a valve such that apressure of <5 torr was made between the sample and the pressure gauge.A leak rate of air through the coupon was recorded over ten minutes.Uncoated SCFS, 1-minute immersion, and 60-minute immersion samplesexhibited leak rates of 31.2 torr/min, 9.34 torr/min. and 0.77 torr/min,respectively.

Example 14

One sample of SCFS was prepared in a similar fashion to Example 13except the sample was suspended over a watch glass of solution such thatonly the face of the coupon was immersed in the solution. A beaker wasplaced over the apparatus to limit solvent evaporation. The sample washeld for 60 minutes and the solution was allowed to infiltrate throughthe porous substrate. The coupon was cured as described in Example 13.This procedure was repeated six times. The sample was then tested forair permeability as described in Example 13. The leak rate wasdetermined to be 1.65 torr/min.

Example 15

One sample of SCFS was prepared in a similar fashion to Example 13except a saturated Kimwipe™ was placed on the surface of the coupon. Abeaker was placed over the apparatus to limit solvent evaporation. Thesample was held for 60 minutes and the solution was allowed toinfiltrate through the porous substrate. The coupon was cured similarlyto Example 13. This procedure was repeated six times. The sample wastested for air permeability as described in Example 13. The leak ratewas determined to be 2.61 torr/min.

Precursor Solution J

A precursor solution was prepared in a manner similar to precursorsolution A by adding aluminum nitrate nonahydrate to anhydrous ethanol(˜375 g/liter). In a separate container, phosphorus pentoxide wasdissolved in anhydrous ethanol (˜284 g/liter) and then the two solutionswere combined and stirred under reflux conditions for 40 hours. Thesolution was concentrated on a rotary evaporator at 65° C. and dilutedto a concentration of 33 g/L. Erbium(III) nitrate pentahydrate (SigmaAldrich, St. Louis, Mo.) was added to the solution with stirring untilfully dissolved. The erbium-phospho-alumina precursor solution had analuminum to phosphorus to erbium atomic ratio of 1 to 1 to 1.5 and aviscosity of 4 cSt.

Example 16

A coupon of slip cast fused silica (SCFS) was coated with precursorsolution G. A coating on the external surface and internal open porewall surfaces was applied using vacuum assisted infiltration ofprecursor solution J. The SCFS substrate first was cleaned by immersingin alcohol while ultrasonicating for 10 minutes, followed by drying inair at 160° C. for 10 minutes and heat cleaning at 800° C. for 60minutes. The substrate was dried at 160° C. and placed in a glass beakerand placed in a vacuum chamber, which was evacuated to a pressure in therange of 0.1-20 torr (0.01-3 kPa) to, with a dwell time under fullvacuum of 10 minutes. The coating precursor solution was introduced intothe vacuum chamber and directed into the beaker holding the SCFSsubstrate through a delivery line. The SCFS sample then was allowed tosoak while completely submerged under the level of the coating precursorliquid for about 60 minutes, while still under vacuum, such that some ofthe alcohol from the precursor solution evaporated. The SCFS substratewas removed from the vacuum chamber and the solution container andpatted dry with a Kimwipe™, and dried at 60° C. for 10 minutes and then160° C. for 10 minutes. Subsequently, the substrate sample was insertedinto a furnace and ramped to 800° C. and a rate of 10° C./min in air andallowed to dwell for 30 minutes to facilitate the curing of theerbium-aluminum phosphate coating. The procedure was repeated anaddition time with precursor solution G, which was diluted to 100 g/L (4cSt).

Example 17

Coated SCFS sample of Example 16 was characterized using transmissionelectron microscopy (TEM) to confirm that the coating was well adheredto the internal SCFS pore surfaces and to show the presence continuouscoverage, high density of the erbium-phospho-alumina coating. A TEMsample was extracted from the central portion of the sample by focusedion beam (FIB) milling. A conformal ˜5 nm denseerbium-aluminum-phosphate coating was observed on a majority of theinner pore wall surfaces of pores ranging from 5-10 nm and on poreranging 200-500 nm. Areas were observed in which aluminum-phosphatelayer (not containing erbium) was observed as an additional layer ofcoating on top of the erbium-aluminum-phosphate coating. Additionally,pores of ˜5-10 nm showed constrictions of fully denseerbium-aluminum-phosphate between the pore walls.

1. A material comprising a porous substrate containing open pores andchannels and comprising interconnected mesopores and micropores, inwhich the pores and channels are internally coated with at least onelayer of phosphorus-containing alumina.
 2. A material of claim 1 inwhich the internal coating layer is at least 5 nm thick.
 3. A materialof claim 1 in which open porosity of a coated substrate is at least 40%of the open porosity of an uncoated substrate.
 4. A material of claim 1in which density an internally coated substrate is increased by no morethan 10% over an uncoated substrate.
 5. A material of claim 1 in whichpore or channel constrictions within the porous substrate are blocked.6. A material of claim 1 in which the internal coating remains stable at1000° C.
 7. A material of claim 1 in which the internal coating layercontains at least two sublayers.
 8. A material of claim 7 in which eachsublayer does not cover all of total surface area within the internalopen pores and channels.
 9. A material of claim 1 in which thephosphorus-containing alumina has aluminum to phosphorus atomic ratiosof 2:1 to 15:1.
 10. A material of claim 1 in which the porous substratecomprises a ceramic material or coating, a ceramic matrix composite, ora metal or metal alloy.
 11. A material of claim 1 in which the poroussubstrate comprises a ceramic thermal barrier coating bonded to a metal.12. A material of claim 1 in which the porous substrate comprises aplasma spray coating, electron beam physical vapor deposition coating,metallurgical overlay coating, or a slurry coating bonded to metal orceramic or ceramic composite.
 13. A material of claim 1 in which theporous substrate comprises graphite; a silicon carbide; a carbon-carboncomposite, a carbon-silicon carbide composite, or a siliconcarbide-silicon carbide composite; or a ceramic refractory or tile. 14.A material of claim 1 bonded to a turbine blade, a vane, or a shroud.15. A method to form a material having an internally coated poroussubstrate surface comprising: a) contacting a porous substratecontaining open pores and channels and comprising interconnectedmesopores and micropores with a non-colloidal, low viscosity solution,having a viscosity lower than 100 centistokes, of aphosphorus-containing alumina coating precursor, formed by combiningsolutions of an aluminum salt and a phosphate ester, such that thecoating precursor infiltrates and internally coats the open pores andchannels; and b) drying and curing the phosphorus-containing aluminacoating precursor to form a layer of phosphorus-containing alumina ontointernal surfaces of the pores.
 16. A method of claim 15 in which apressure differential is applied to the porous substrate by placing thesubstrate under vacuum conditions before contacting with the coatingsolution.
 17. A method of claim 15 in which the alcohol is methanol,ethanol, or a mixture thereof.
 18. A method of claim 15 in which stepsa) and b) are repeated at least twice.
 19. A method of claim 15 in whichphosphorus-containing alumina has an aluminum to phosphorus atomic ratioof 4 to
 10. 20. A method to protect a porous substrate surfacecontaining open pores from environmental conditions comprising: a)contacting a porous substrate containing open pores and channels andcomprising interconnected mesopores and micropores under externalpressure conditions with a non-colloidal, low viscosity solution of aphosphorus-containing alumina coating precursor, formed by combiningsolutions of an aluminum salt and a phosphate ester, such that thecoating precursor infiltrates and internally coats the open pores; andb) drying and curing the phosphorus-containing alumina coating precursorto form a continuous layer of phosphorus-containing alumina ontointernal surfaces of the pores.